many_many

<?php

class PostPage extends Page {

  public static $db = array(
    'Title' => 'Text'
  );

  public static $many_many = array(
    'Authors' => 'AuthorPage'
  );

  public function getCMSFields() {

    $fields = parent::getCMSFields();


    // Get all Campuses (locations)
    $campuses = AuthorPage::get();

    // Create a map(array) with the format array(ID=>Title). This is the default as per
    // http://api.silverstripe.org/3.0/class-DataList.html#_map
    $map = $campuses->map();

    // Create a CheckboxSetField to select course locations
    $courseLocations = new CheckboxSetField('Authors', 'Choose course locations', $map);

    // Add the CheckboxSetField to the CMS
    $fields->addFieldToTab('Root.Main', $courseLocations, 'Content');
    $fields->addFieldToTab('Root.Main', new TextField('Title'), 'Content');


    return $fields;
  }
}

class AuthorPage extends Page {

  public static $db = array(
    'Name' => 'Text'
  );

  public static $many_many = array(
    'Posts' => 'PostPage'
  );

  public function getCMSFields() {

    $fields = parent::getCMSFields();

    $fields->addFieldToTab('Root.Main', new TextField('Name'), 'Content');


    return $fields;
  }
}

class HomePage_Controller extends Page_Controller {

  public function filterStuff() {
    $results = PostPage::get()->filter(array('Authors.ID:ExactMatch' => 1));
    return $results->renderWith('SomeTemplate');
  }
}